1. Field of the Invention
This invention relates generally to image display devices and is concerned more particularly with a device having light emitting means disposed for displaying an image.
2. Discussion of the Prior Art
An image display tube may be of the cathode ray type having a generally funnel-shaped envelope terminating at its larger end in an image transmissive, output faceplate. Disposed adjacent the inner surface of the output faceplate is an aligned imaging screen comprising a layer of phosphor material which fluoresces locally when a discrete region thereof is penetrated by beamed electrons. As a result, the discrete region emits light with an intensity or brightness corresponding to the energies expended by the penetrating electrons.
In operation, an electron beam is directed onto the inner surface of the imaging screen and deflected laterally over it to move into alignment with respective discrete regions of the phosphor material. Also, the instantaneous value of the electron current in the beam is varied to cause the respective discrete regions of the phosphor material to emit light with varying intensity. As a result, the imaging screen produces a visible light image having discrete areas of varying light intensity aligned with the respective discrete regions of the phosphor material penetrated by electrons. The visible light image, thus produced, is directed through the output faceplate to emerge from the tube for external viewing.
The visible light image transmitted through the output faceplate also may be required to pass through a light transmissive implosion panel which generally is bonded to the exterior surface of the output faceplate. For safety purposes, the implosion panel usually is relatively thick, such as one-eighth of an inch, for example. In practice it may be found that the implosion panel and the output faceplate permit ambient light to enter the tube and impinge on the imaging screen. As a result, the entire phosphor layer of the imaging screen may scatter a background light which degrades contrast in the visible light image produced for external viewing. Consequently, image display tubes of the prior art may be provided with an output assembly for supporting a layer of light-filtering material in alignment with the exterior surface of the output faceplate. The material of this light-filtering layer readily transmits light of the color or wavelength produced by the phosphor material of the imaging screen and selectively absorbs light of other colors or wavelengths, thereby enhancing contrast in the image produced for external viewing.
The output assembly for supporting the light-filtering layer usually comprises a sandwich of two implosion panels having the light-filtering layer between them. It is generally held that the two implosion panels are necessary to provide the interposed layer of light-filtering material with smooth surfaces and a substantially uniform thickness so that the integrity of shading in the image is not disturbed or distorted by irregularities in the light-filtering material. Thus, due to the thicknesses of the implosion panels, the light-filtering sandwich of implosion panels increases the weight of the tube significantly. Furthermore, because space in front of the output faceplate is limited in some installations, the implosion panel sandwich generally is required to have a curvature conforming to the curvature of the output faceplate of the tube and is bonded directly to the exterior surface of the output faceplate. However, the two implosion panels may not be readily available for forming a sandwich with the proper curvature and having the desired image transmitting properties.